Method for blow molding hollow articles in minimum cycle time

ABSTRACT

A method of making a hollow plastic article characterized by a low cycle time. A mold is cooled to a selected temperature below ambient, a tube of plastic material open at its leading end is extruded to a length greater than the length of the article to be formed, and the tubing is enclosed in the mold at a mold closing rate such that condensation has not formed in the mold. During the closing of the mold, the leading end of the tubing is closed and sealed to entrap air at atmospheric pressure and a sufficient portion of the leading end of the sealed tube is collapsed by the closing mold to reduce the internal volume of the tubing and partially expand the tubing due to the resulting increase of internal pressure. Air is then injected into the partially expanded tubing to inflate the tubing to final form in the closed mold and the mold is then opened and the finished article rejected in the minimum cycle time comensurate with the selected temperature to which the mold is cooled.

United States Rupert et al.

atent [72] Inventors: Samuel J. Rupert; William E. Ziegler, both of AnnArbor, Mich.

Hoover Ball and Bearing Company, Saline, Mich.

[73] Assignee:

FOREIGN PATENTS OR APPLICATIONS 375,737 6/1962 Japan ..264/94 630,5817/1963 Belgium..... ....264/94 1,085,662 9/1963 Germany .....264/96Primary Examiner-Robert F. White Assistant Examiner.l. H. Silbaugh [57]ABSTRACT A method of making a hollow plastic article characterized by alow cycle time. A mold is cooled to a selected temperature belowambient, a tube of plastic material open at its leading end is extrudedto a length greater than the length of the article to be formed, and thetubing is enclosed in the mold at a mold closing rate such thatcondensation has not formed in the mold. During the closing of the mold,the leading end of the tubing is closed and sealed to entrap air atatmospheric pressure and a sufficient portion of the leading end of thesealed tube is collapsed by the closing mold to reduce the internalvolume of the tubing and partially expand the tubing due to theresulting increase of internal pressure. Air is then injected into thepartially expanded tubing to inflate the tubing to final form in theclosed mold and the mold is then opened and the finished articlerejected in the minimum cycle time comensurate with the selectedtemperature to which the mold is cooled.

3 Claims, 10 Drawing Figures 2 Sheets-Sheet 1 INVENTORS SAMUEL J. RUPERTWILLIAM E. ZIEGLER ATTORNEYS Patented May 23, 1972 2 Sheets-Sheet 2 FIGSINVENTORS SAMUEL J. RUPERT BY WILLIAM E. ZIEGLER flaw! ATTORNEYS METHODFOR BLOW MOLDING HOLLOW ARTICLES IN MINIMUM CYCLE TIME This applicationis a continuation of application Ser. No. 722,978, filed Apr. 22,1968,and now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to blowmolding of hollow articles from organic plastic materials, andparticularly is directed to a method of forming hollow articles inminimum cycle time.

It is well known in the art of blow molding to fonn a parison which canbe sealed and partially inflated prior to being enclosed in a mold.Generally, the purpose of such prior art devices is to change the shapeof the parison from a straight extruded tube to one which conforms morenearly to the configuration of the product which is to be formed. Insome instances these prior art practices are followed in order to permitextruding the parison through a relatively small orifice, which is anextrusion operation easier to perform than when larger diameter orificesare employed, and it is also done to permit the combined injectionmolding of the neck finish with a subsequent blow molding process.

The prior art procedures described above have a serious drawback in thatthey increase the cycle time, thereby reducing the overall efficiency ofthe blow molding apparatus. One of the principal objects in blow moldingis to produce containers having the lowest unit cost, and this demandsthat the cycle time be as short as is possible. When the shaping of theparison occurs during mold-open time, the normal result is that anincrease in mold-open time is required, and this in turn also has theharmful effect of increasing the mold-close time. This follows becausethe mold-close time is indirectly influenced by the length of time thatthe mold is open. The mold-close time is determined by the rate of heattransfer of the plastic material to the walls of the mold, and thecolder the mold, the faster the transfer of heat, and therefore, theshorter the mold-close time. For this reason, it is conventionalpractice to pass coolants through the molds to reduce the temperature ofthe molds below the ambient temperature. The extent to which the moldscan be cooled is limited by the ambient humidity conditions and the timethat the molds are left open. The longer that the molds are left open,the more nearly the mold temperature must approach the ambienttemperature, and conversely, the shorter the mold-open time, the greatercan be the temperature differential between the molds and the ambientatmosphere. This condition prevails because if the molds are reduced toan excessively low temperature or are left open for an excessive timeperiod, condensation will form in the mold faces which, after the moldsare closed and the articles are blown will impair the surfaces of thearticles to destroy or lower their usefulness.

Thus it follows that the shorter the mold-open time, the lower is thetemperature of the molds that can be used, and this in turn will havethe effect of shortening the mold-close time, and therefore, the overallcycle time of the blow molding operation will be kept at a minimum.Therefore, in order to operate in the most efficient manner, it isdesirable to form an acceptable and useable parison by selecting anddesigning the proper extrusion head tooling rather than to rely onchanging the shape of the extruded parison after its extrusion butbefore the molds have been closed. However, this more desirableprocedure may also create additional problems. For example, when makinga large lightweight container having a hollow handle, the parison musthave a relatively large diameter, and accordingly is very thin whenextruded, and therefore, it does not have a great amount of resistanceto change in its shape. When a mold is closed on such a large,thin-walled parison, there is a tendency for the sides of the parison tocome together thus creating a web in the final container. These websnormally occur in the handle portion of the container, and when thisoccurs, the usefulness of the container may be im paired, because thefilling operation will be affected so that in many instances a containerhaving webbing in the handle may be unuseable. This is particularly truein the dairy industry where milk containers must be completely filledand cannot have voids in the handles, for example.

SUMMARY OF THE INVENTION The present invention has overcome theshortcomings of the prior art and provides an improved method for blowmolding articles from plastic material, wherein extrusion orifices ofdesired size or shape are used which have been selected to provide themost efiicient cycle time for the blow molding operation. By virtue ofthe present invention this selection can be made without impairment ofthe quality of the finished product. This has been accomplished byselecting a die head having an extrusion orifice of the dimensionnecessary for blow molding in the most efiicient manner, and mold meansare provided which function when closing the mold on the extrudedparison to initially close the open end of the extruded parison, andthereafter to pinch closed an additional portion of the parison outsidethe mold cavity so as to partially inflate the parison confined withinthe cavity to assure that the enclosed parison is free of anyundesirable webbing which might be formed if a partial collapse of thetube were to occur when the mold sections moved into closed positions.Thereafter the parison is blown to its final form within the mold.

In accordance with a preferred form of the present invention, a methodof blow molding is provided which utilizes apparatus comprising acombination of extrusion means for extruding a tube through an annularorifice, mold means for enclosing the extruded tube in an articleforming cavity to form said blank, and means for injecting a gaseousmedium into the blank to expand it to the shape of the cavity. The moldmeans comprises a pair of separable mold sections defining the cavity,and each of the sections has an extensible pinch bar seated in itsparting face at a location spaced from the cavity portion. Each pinchbar is in abutting relation to the pinch bar of the other section whensuch sections are closed. Means are provided for urging the pinch barsto extended positions outward of the parting faces when the moldsections are open and operable when the mold sections are moved towardclosed positions to allow the pinch bars to come into abuttingengagement and thereafter to return to seated positions as an incidentof the closing of the mold sections. In such an operation, the molds canbe cooled to the maximum extent permitted below ambient temperatures,and the mold-open time is kept at a minimum, being restricted only tothe length of time that it is required to extrude the parison.

Thus, the present invention includes the method of making a plasticarticle by blow molding a blank in an enclosed mold comprising the stepsof cooling the mold to a selected temperature below ambient temperature,extruding a tube of plastic material open at its leading end, andclosing the tube in the mold at a rate of time such that significantcondensation has not yet formed in the mold, and as an incident toenclosing the tube in said mold the open end of the tube is closed toform a blank with air trapped therein substantially at atmosphericpressure and a portion of the closed blank is then collapsed to reducethe volume of the blank so as to increase the internal air pressure toexpand the blank partially as the mold is closed. Thereafter, air isinjected into the partially expanded blank to inflate the blank to finalform.

Thus, it is the primary object of the present invention to provideimproved method for producing plastic articles,

Other objects of this invention will appear in the following descriptionand appended claims, reference being had to the accompanying drawingsforming a part of this specification wherein like reference charactersdesignate corresponding parts in the several views.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmentary view, partlyin section, showing the cavity of one mold section and the associatedextrusion head of the blow molding machine;

FIG. 2 is a fragmentary side elevational view showing the mold sectionsin closed positions and showing in section the means for pinching theextruded tube together prior to complete closing of the mold sections;

FIGS. 3, 4, 5 and 6 are fragmentary sectional views taken through thetwo mold sections showing in sequence the extrusion of the tube in theopen mold sections, the closing of the tube prior to complete closing ofthe mold sections, pinching closed a portion of the closed blank topartially inflate the blank remaining within the mold cavity, andblowing of the blank to its final form; and

FIGS. 7, 8, 9 and 10 show similar steps of operation when viewed on aplane passing through the center line of the extrusion head parallel tothe parting surfaces of the mold sections.

DESCRIPTION OF THE PREFERRED EMBODIMENT Before explaining the presentinvention in detail, it is to be understood that the invention is notlimited in its application to the details of construction andarrangement of parts illustrated in the accompanying drawings, since theinvention is capable of other embodiments and of being practiced orcarried out in various ways. Also, it is to be understood that thephraseology or terminology employed herein is for the purpose ofdescription and not of limitation.

Referring now to the drawings, the invention will be described ingreater detail. Initially the apparatus illustrated in FIGS. 1 and 2will be described, and attention is directed to these two figures. Aconventional extrusion die head 10 is provided with an outlet orifice 12through which the plastic material 14 can be extruded in the form of anopen-ended tube 16. Air can be introduced into the extruded tube 16through the conventional blow pin 18. A conventional stripper member 20is provided for stripping the blown container from the die head 10 afterthe article has been blown. For more details of this conventionalapparatus, reference is made to prior U.S. Pat. No. 3,120,679, grantedFeb. 11, 1964 to C. A. Price et al.

Positioned adjacent to the extrusion die head 10 is the mold 22 whichincludes the mold sections 24 and 26 which are separable on their commonparting faces or surfaces 28. The mold sections form the cavity 30 inwhich can be blow molded a jug having a hollow handle. For this purpose,the mold sections are provided with lands 32 which engage oppositesurfaces of the extruded tube 16 during closing of the mold sections,thereby forming a blank which has a main body portion and a tubularportion in the handle portion 34 of the mold sections. As is describedin further detail in U.S. Pat. No. 3,120,679, during the subsequentblowing step, the jug will be formed having the hollow handle defined bythe handle portion 34.

When blow molding containers in the form of jugs having a capacity ofone gallon or more, it is normally necessary to extrude a tube 16 havinga relatively large diameter, substantially equal to the maximumtransverse dimension of the container, if such jug is to have a blownhollow handle. The wall thickness of the extruded tube will then berelatively small, and it is found that under these circumstances theportion of the extruded tube which is confined in the closed mold 22 inthe handle portion 34 will have relatively little resistance tocollapse, and if any collapse should occur, webbing will be createdacross the hollow handle when the subsequent blowing operation takesplace.

In order to avoid this problem, the lower portions of the mold sections24 and 26 incorporate improvements which will permit partial inflationof the parison at the time the mold sections are closed, therebypreventing the creation of webbing in the handle. For this purpose, eachof the mold sections has an extensible pinch bar 36 which is seated inthe parting face or surface at a location spaced below the bottom edgeof the cavity 30 so that a relatively flat land 38 is provided betweenthe pinch bar and the cavity 30. Each pinch bar 36 is mounted on apiston 40 which is adapted to reciprocate in a cylindrical housing 42.Suitable bushings 44 may be provided to facilitate easy movement of thepiston 40 therein. The inner end of the piston 40 has an enlarged head46 to limit the forward movement of the piston when it comes intoengagement with the shoulder 48. The piston 40 can be moved so that thepinch bar 36 is in an extended position when air under pressure isintroduced through the conduit 50 into the inner end of the cylindricalhousing 42.

The pinch bars 36 are arranged so that they will be in abuttingpositions when the mold sections are closed. When the mold sections aremoved toward a closed position, the pinch bars 36 will engage oneanother and further movement of the mold sections to the closed positionwill result in the pistons 40 being moved to a retracted positionagainst the low air pressure to the positions shown in FIG. 1. It is tobe understood that when the pinch bars 36 are in the fully extendedpositions, and the mold sections 24 and 26 are in their open positions,a space will exist between the pinch bars 36 for passage of the extrudedtube 16. When the mold sections are moved toward their closed positions,air will be introduced into the cylindrical housing 42 into the conduits50 to extend the pinch bars, and initially the pinch bars will engagethe lower portion of the extruded tube to close the open end, and thenas the mold sections continue to move toward their closed positions, thelower portion of the closed blank will be engaged by the faces 38 of themold sections, thereby closing an additional portion of the lower end ofthe extruded tube, and the air which was in the closed portion will beforced into the remaining open portion of the blank parison. This willhave the effect of partially expanding the blank or parison. Thus, asthe molds close the portion of the parison which is in the handleportion 34 of the mold sections will be partially inflated, therebypreventing partial collapse of this portion of the parison. Thereafter,the parison can be fully inflated by introduction of air through the airmandrel l8, and a container will be blown to final form which is free ofany imperfections, such as undesirable webbing that might otherwise bepresent in the hollow handle of the jug.

Referring now to FIGS. 3-10, a brief description will be provided of thesequence of steps that occur in this operation. As shown in FIGS. 3 and7, a hollow open-ended tube 16 has been extruded between the open-moldsections 24 and 26. The mold sections are next moved toward closedpositions and in timed relation with this movement, the pistons 40 aremoved to extended positions of the pinch bars 46, which acts to closethe lower end of the extruded tube. This step is shown in FIGS. 4 and 8.As the mold sections 24 and 26 continue movement to a closed position, afurther portion of the parison is pinched between the faces 38 to reducethe total volume of the extruded parison or blank, thereby causing theblank to partially expand as can be seen best in FIGS. 5 and 9. Thisfurther closing of the parison has in effect created a puffing actionwithin the parison which will protect the parison against anyundesirable forming of webbing within the finished product. The finalmolding step of the operation is shown in FIGS. 6 and 10 whichillustrate the shape of the article in its final form. Subsequently, themold sections can be open and the stripper mechanism 20 (FIGS. 1 and 2)can be lowered to break the blown container away from the die head 10.

It is to be understood that the mold sections 24 and 26 are constructedin a conventional manner to have cooling water circuits or the like,therein through which a coolant can pass in order to maintain the moldsections at a desired temperature. By virtue of the invention described,the mold sections will remain open the minimum time required to extrudethe parison 16, after which the molds can be closed and as an incidentto closing of the molds, the lower end of the parison or tube is closed,and immediately thereafter, as the mold sections come together, the airwhich has been trapped within the parison, substantially at atmosphericpressure, will be compressed into a smaller volume by closing of thefaces 38 upon the lower portion of the parison, thereby increasing thepressure of the air in the remaining parison so as to partially inflatethe parison within the cavity 30. The parison can then be blown to itsfinal form, and it will be recognized by those skilled in the art, thatno lost time has occurred resulting from preshaping or in any other wayof working on the parison prior to closing the mold sections which wouldoperate to delay the closing of such sections.

It is claimed:

1. A method of making a hollow plastic article by preshaping and blowinga blank in an enclosing mold wherein the method is characterized by thelow cycle time required to form the article free of surface impairmentscaused by condensation on the mold cavity surfaces, comprising the stepsof cooling said mold to a selected temperature below ambienttemperature, extruding a tube of plastic material open at its leadingend to a length greater than the corresponding dimension of the hollowarticle to be blown, enclosing the tube in said mold at a rate of timesuch that significant condensation corresponding to said selectedtemperature has not yet formed in said mold, and as an incident toenclosing the tube in said mold and without increasing the mold closingtime (a) closing the leading end of the tube to form a blank with airtrapped therein substantially at atmospheric pressure and (b) collapsinga sufficient portion of the leading end of the closed blank to reducethe volume thereof so as to increase the internal air pressure to expandthe blank partially substantially simultaneously with completion of theenclosing of the blank in the mold, injecting air into the partiallyexpanded blank to inflate the blank to final form, and opening the moldto discharge the blown article at a minimum time period commensuratewith the mold having been cooled to said selected temperature.

2, In a method of making a plastic article having an integral hollowhandle by blowing a blank in an enclosing mold wherein the enclosingmold when in its closed position has a pair of abutting lands fordefining the opening in the handle portion of the article and whereinthe method is characterized by the low cycle time required to form thearticle free of surface impairments caused by condensation on the moldcavity surfaces, comprising the steps of cooling said mold to a selectedtemperature below ambient temperature, extruding a tube through anorifice to a length greater than the corresponding dimension of thearticle to be blown, initiating the closing of the mold about theextruded tube, closing the leading end of the extruded tube as aninitial incident to closing of the mold to provide a closed blankcontaining air at approximately atmospheric pressure, immediatelythereafter completing enclosing said blank in said mold and pinchingtogether an intermediate portion of the blank at said lands and asufficient portion of the leading end of the blank so as to transfer theair in the leading end to the remainder of the blank wherein the handleis to be formed to inflate said remainder partially substantiallysimultaneously with completion of closing of the mold to avoid formationof webs in said handle, said closing of the mold being at a rate of timesuch that significant condensation corresponding to said selectedtemperature has not yet formed in said mold, then injecting a gaseousmedium into said blank to inflate the blank to final form, and openingthe mold to discharge the blown article at a minimum time periodcommensurate with the mold having been cooled to said selectedtemperature 3. In making a plastic container having a hollow handle freeof internal webbing by blowing a blank in an enclosing mold whichdefines the container wherein the method is characterized by the lowcycle time required to form the article free of surface impairmentscaused by condensation on the mold cavity surfaces, the steps of coolingsaid mold to a selected temperature below ambient temperature, extrudinga tube through an orifice to a length greater than the correspondingdimension of container to be blown, initiating the closing of the moldabout the extruded tube, closing the leading end of the extruded tube asan initial incident to closing the mold to provide a closed blankcontaining air a proximately at atmosphenc pressure, immediately thereater completing enclosing said blank in said mold and pinching together asufficient portion of the leading end of the blank so as to transfer theair therein to the remainder of the blank to create a pufiing effectwithin said remainder simultaneously with closing of the mold so as toprevent formation of webbing in the handle of the blank which mightotherwise be caused by the closing of the mold, said closing of the moldbeing at a rate of time such that significant condensation correspondingto said selected temperature has not yet formed in said mold, theninjecting air into the enclosed blank to inflate the blank to finalform, opening the mold to discharge the blow article at a minimum timeperiod commensurate with the mold having been cooled to said selectedtemperature.

1. A method of making a hollow plastic article by preshaping and blowinga blank in an enclosing mold wherein the method is characterized by thelow cycle time required to form the article free of surface impairmentscaused by condensation on the mold cavity surfaces, comprising the stepsof cooling said mold to a selected temperature below ambienttemperature, extruding a tube of plastic material open at its leadingend to a length greater than the corresponding dimension of the hollowarticle to be blown, enclosing the tube in said mold at a rate of timesuch that significant condensation corresponding to said selectedtemperature has not yet formed in said mold, and as an incident toenclosing the tube in said mold and without increasing the mold closingtime (a) closing the leading end of the tube to form a blank with airtrapped therein substantially at atmospheric pressure and (b) collapsinga sufficient portion of the leading end of the closed blank to reducethe volume thereof so as to increase the internal air pressure to expandthe blank partially substantially simultaneously with completion of theenclosing of the blank in the mold, injecting air into the partiallyexpanded blank to inflate the blank to final form, and opening thE moldto discharge the blown article at a minimum time period commensuratewith the mold having been cooled to said selected temperature.
 2. In amethod of making a plastic article having an integral hollow handle byblowing a blank in an enclosing mold wherein the enclosing mold when inits closed position has a pair of abutting lands for defining theopening in the handle portion of the article and wherein the method ischaracterized by the low cycle time required to form the article free ofsurface impairments caused by condensation on the mold cavity surfaces,comprising the steps of cooling said mold to a selected temperaturebelow ambient temperature, extruding a tube through an orifice to alength greater than the corresponding dimension of the article to beblown, initiating the closing of the mold about the extruded tube,closing the leading end of the extruded tube as an initial incident toclosing of the mold to provide a closed blank containing air atapproximately atmospheric pressure, immediately thereafter completingenclosing said blank in said mold and pinching together an intermediateportion of the blank at said lands and a sufficient portion of theleading end of the blank so as to transfer the air in the leading end tothe remainder of the blank wherein the handle is to be formed to inflatesaid remainder partially substantially simultaneously with completion ofclosing of the mold to avoid formation of webs in said handle, saidclosing of the mold being at a rate of time such that significantcondensation corresponding to said selected temperature has not yetformed in said mold, then injecting a gaseous medium into said blank toinflate the blank to final form, and opening the mold to discharge theblown article at a minimum time period commensurate with the mold havingbeen cooled to said selected temperature.
 3. In making a plasticcontainer having a hollow handle free of internal webbing by blowing ablank in an enclosing mold which defines the container wherein themethod is characterized by the low cycle time required to form thearticle free of surface impairments caused by condensation on the moldcavity surfaces, the steps of cooling said mold to a selectedtemperature below ambient temperature, extruding a tube through anorifice to a length greater than the corresponding dimension ofcontainer to be blown, initiating the closing of the mold about theextruded tube, closing the leading end of the extruded tube as aninitial incident to closing the mold to provide a closed blankcontaining air approximately at atmospheric pressure, immediatelythereafter completing enclosing said blank in said mold and pinchingtogether a sufficient portion of the leading end of the blank so as totransfer the air therein to the remainder of the blank to create apuffing effect within said remainder simultaneously with closing of themold so as to prevent formation of webbing in the handle of the blankwhich might otherwise be caused by the closing of the mold, said closingof the mold being at a rate of time such that significant condensationcorresponding to said selected temperature has not yet formed in saidmold, then injecting air into the enclosed blank to inflate the blank tofinal form, opening the mold to discharge the blow article at a minimumtime period commensurate with the mold having been cooled to saidselected temperature.